In prior art low frequency circuits, the variation of data bits relative a local oscillator typically was not great enough to cause problems as long as the cable length was of a substantially fixed value. As the frequency of data being transferred rose, and especially if cable lengths between remote and local circuits were great enough or varied, means had to be designed for keeping the frequency of a remote data source tied to the frequency of the local clock and arrangements needed to be made so that the clocking of data at the local source always coincided in a given window with the local clock such that bit errors did not occur due to violation of set up and hold time specifications of local circuits.
One approach used in the prior art was to transmit a clock from the local source to the remote source for use in clocking the data signal. Both the data signal and the clock at the remote source were then returned to the local source, thus requiring three transmission lines. Both the returned clock and the returned data transmission lines needed to be of substantially identical length, otherwise temperature and other parameters could still cause timing problems. The returned clock would then be used in the local circuit for further data processing.
The present invention adds a further slave oscillator and phase detection circuit but eliminates the expense of a third transmission line by having the slave oscillator, which comprises a part of a phase lock loop that operates in frequency the same as a local master oscillator and holds a phase relative incoming data to an amount, such that the local master clock is continuously and automatically centered relative the incoming data.
It is thus an object of the present invention to provide an improved data synchronizing circuit.